1. Field of the Invention
The present invention relates to dielectric ceramic compositions having high dielectric constants and ceramic electronic devices using the dielectric ceramic compositions as dielectric layers.
2. Description of the Related Art
Recently, significant improvements have been made in the performance of electronic devices. In particular, in information processing apparatus such as computers, mobile communication terminals, and the like which have led the new, information-oriented society, higher processing speeds, miniaturization, enhanced multifunctionality, and the like have been actively pursued. Highly integrated and functional semiconductor devices provided with higher processing speeds, such as VLSI and ULSI, have primarily been responsible for improvements in information processing apparatuses. However, even though the speed and performance of semiconductor devices have improved, signal delay, cross talk, impedance mismatch, noise generation due to fluctuation in power supply voltages, and the like may occur on substrates where devices are connected to each other, and hence, the system performance may be limited, that is, the potential performance of semiconductor devices is not fully exploited in some cases.
Accordingly, a so-called multichip module (MCM) has been practically used as a substrate for performing reliable information processing at a higher speed. In an MCM, a plurality of semiconductor devices are mounted on a ceramic substrate. In order to increase the mounting density of the semiconductor devices and to electrically interconnect them reliably, ceramic multilayer substrates in which conductive patterns are disposed in three-dimensional arrangements are particularly useful as MCM substrates.
Alumina has been conventionally used as an insulating material for the ceramic multilayer substrate described above. However, the baking temperature of alumina is a high temperature of not less than 1,500xc2x0 C., and as a result, a high melting point metal, such as tungsten or molybdenum, must be used to make the conductors on the substrate so that simultaneous baking with alumina can be performed. In addition, the high melting point metals mentioned above are susceptible to oxidation, and hence, baking must be performed in a reducing atmosphere. Furthermore, since the high melting point metals have high resistivities, the ceramic multilayer substrate has a limitation in, particularly, its high frequency characteristics.
In general, alumina has a high relative dielectric constant of approximately 10, so that signal delay may be larger in some cases when semiconductor devices are operated at high speeds. Alumina also has a high coefficient of thermal expansion compared to that of silicon, which is frequently used for semiconductor devices, whereby degradation of reliability caused by thermal cycles may occur in some cases.
Accordingly, in order to solve the problems described above, the use of low-temperature sinterable ceramic materials, which are composites of ceramic components and glass components, has been aggressively researched, and practical use of ceramic multilayer substrates formed of the composites mentioned above has been pursued. The low-temperature sinterable ceramic material is a material comprising a ceramic component as a mother material and a glass component as a sintering auxiliary agent. Since the low-temperature sinterable ceramic material has a low sintering temperature, freedom in material characteristics and baking temperatures can be significantly broadened. In particular, when the low-temperature sinterable ceramic material is used, a low-melting point metal, such as a copper-based, a silver-based, or a gold-based metal, each having low resistivity, can be simultaneously baked, and hence, a ceramic multilayer substrate having superior frequency characteristics can be formed.
Recently, research has been performed, in which passive devices, such as capacitors and inductors, which are constituents of devices to be mounted on substrates, are embedded in a ceramic multilayer substrate so as to realize further miniaturization of modules. However, in the case in which the passive devices are embedded in the ceramic multilayer substrate, when characteristics of the passive devices embedded in the substrate are inferior to those mounted on the surfaces of the substrate, the advantages thereof are reduced by half. Hence, the characteristics of the passive devices embedded in the substrate must be equivalent or superior to those mounted on the substrate.
Accordingly, when passive devices are embedded in the ceramic multilayer substrate, a material for the substrate is, in general, properly selected so that characteristics of the passive devices can be fully utilized. For example, portions at which capacitors are formed are composed of dielectric layers having high dielectric constants, and other portions are composed of insulating layers having high resistances, so that compact and improved ceramic multilayer substrates can be obtained.
The applicant of the present invention has disclosed a dielectric ceramic composition having a high dielectric constant which can be used in a dielectric layer, in Japanese Unexamined Patent Application Publication No. 8-45347, which is represented by the general formula Ba{(CoxZn1xe2x88x92x)yNb1xe2x88x92y}zOw, in which, on a molar basis, 0 less than x less than 1, 0.313xe2x89xa6y less than 0.333, 0.993xe2x89xa6z less than 1, and w is an optional number. Even though the dielectric ceramic composition is a composition obtained by baking at a high temperature of 1,420 to 1,520xc2x0 C., the composition can be baked in a relatively short time and has superior electrical characteristics such that the Q value is not less than 10,000 at approximately 7 GHz.
However, since the dielectric ceramic composition described in Japanese Unexamined Patent Application Publication No. 8-45347 has a high sintering temperature of not less than 1,420xc2x0 C., simultaneous baking with a low melting point metal, such as silver or copper, cannot be performed. When a glass component is added to the dielectric ceramic composition so as to decrease the baking temperature, depending on the types and amounts added, substrate strength may be significantly decreased compared to that of an alumina substrate, or its electrical characteristics and/or temperature characteristics may be significantly decreased in some cases even though the substrate strength is high.
Specifically, when substrate strength is regarded as important, the relative dielectric constant is small, so that capacitors to be embedded in the substrate are unlikely to have high capacitance. In order to provide capacitors having high capacitance, the electrode areas occupied thereby must be large, so that it is difficult to realize substrate miniaturization and to increase surface mounting densities. In contrast, when electrical characteristics and temperature characteristics are regarded as important, the mechanical strength is low, so that the reliability of the substrate may be poor if used as a substrate for mounting semiconductors and the like.
Accordingly, taking the conventional problems into consideration, the present invention provides a low-temperature sinterable dielectric ceramic composition having superior electrical and temperature characteristics and a high dielectric constant, and a ceramic electronic device using the same.
In one aspect of the present invention, there is provided a dielectric ceramic composition comprising a dielectric ceramic component represented by the general formula Ba{(CoxZn1xe2x88x92x)yNb1xe2x88x92y}zOw, in which, on a molar basis, 0 less than x less than 1, 0.313xe2x89xa6y0.333, 0.993xe2x89xa6z less than 1, and w is an optional number, and a glass component comprising at least silicon oxide and boron oxide, wherein the dielectric ceramic component and the glass component are mixed.
In the dielectric ceramic composition of the present invention, 1 to 25 parts by weight of the glass component may be mixed with 100 parts by weight of the dielectric ceramic component.
In the dielectric ceramic composition of the present invention, the glass component may comprise 10 to 60 percent by weight of silicon oxide, 5 to 40 percent by weight of boron oxide, 0 to 30 percent by weight of aluminum oxide, 20 to 70 percent by weight of at least one of an alkaline earth oxide and zinc oxide, and 0 to 15 percent by weight of an alkali metal oxide.
The dielectric ceramic composition of the present invention may further comprise cerium oxide in a ratio of 0.5 parts by weight of the cerium oxide to 100 parts by weight of the dielectric ceramic component.
In another aspect of the present invention, there is provided a ceramic electronic device comprising a dielectric layer and a conductive layer provided thereon, wherein the dielectric layer is composed of the dielectric ceramic composition of the present invention.
In the ceramic electronic device of the present invention, the conductive layer comprises at least one conductive material selected from the group consisting of a copper-based, a silver-based, and a gold-based conductive material.
Since the dielectric ceramic composition of the present invention is composed of the dielectric ceramic component represented by the general formula described above and the glass component (hereinafter referred to as SiO2xe2x80x94B2O3-based glass component) containing at least silicon oxide and boron oxide mixed with the dielectric ceramic component, a high relative dielectric constant, a small temperature coefficient of resonant frequency, and a high Q value in a high frequency region of the dielectric ceramic component can be maintained. In addition, the dielectric ceramic composition having superior electric and temperature characteristics described above can be obtained by low-temperature sintering at not more than a melting point of a low-melting point metal.
In the ceramic electronic device of the present invention having the dielectric layer provided with the conductive layer thereon, since the dielectric layer is composed of the dielectric ceramic composition of the present invention, simultaneous baking can be performed with a conductive low-melting point metal material having a low resistivity, and as a result, a ceramic electronic device can be obtained which has superior electrical and temperature characteristics, and more specifically, superior high frequency characteristics.
The above and other features and advantages of the present invention will be apparent from the following detailed description of embodiments of the invention in conjunction with the accompanying drawings in which like references denote like elements and parts.